So, I just bought one of these from Amazon -- cute little unit. It's fun-sized, which is the way I like things, since I'm fun-sized myself. The sound quality from it is fine, for what I paid (about $65 after shipping); nothing outstanding but it does get quite loud powering my two ancient -- in technology terms -- Optimus PRO-7AV speakers.

I'm having a little problem with it, though, where the left channel cuts out when headphones are unplugged from the device. If you raise the volume it will come back on, and it stays on when you lower the volume, but when playing at low volume, the left channel will eventually cut out again. This seems indicative of some greater problem with the unit; does that sound correct?

I'm considering returning it anyway, for another problem; with headphones plugged in, the output from the speakers is still quite loud (even though it does get significantly muted, you have to turn the volume way up for the headphones to play with any volume -- much louder than I would choose to run it for the speakers).

Very curious to hear your thoughts on this matter.

If you're curious, here's the unit: (I had a gift certificate.)http://www.amazon.com/Pyle-PTA4-2x120-Stereo-Amplifier/dp/B003NVN1PY/ref=sr_1_1?ie=UTF8&qid=1360287726&sr=8-1

Captain Ned wrote:Well, the one EBay review talks about a weak or dead left channel. I'm assuming that was you.

Wasn't, actually. Haha. Hmm.

Captain Ned wrote:120WPC (with no distortion spec) at 4Ω for under $100 does not speak well of the components in the box.

Yeah, I guess so. It still sounds better than my old Kenwood KA-3500 that my dad gifted me. :) I guess I'll keep it and just live with the issues.

I should probably fiddle with the speaker wires; I noticed last night that rather than cranking up the volume, I can mess with the speaker wires and it seems to make the left channel come back on. Kinda strange.

Captain Ned wrote:Well, the one EBay review talks about a weak or dead left channel. I'm assuming that was you.

Wasn't, actually. Haha. Hmm.

Captain Ned wrote:120WPC (with no distortion spec) at 4Ω for under $100 does not speak well of the components in the box.

Yeah, I guess so. It still sounds better than my old Kenwood KA-3500 that my dad gifted me. I guess I'll keep it and just live with the issues.

I should probably fiddle with the speaker wires; I noticed last night that rather than cranking up the volume, I can mess with the speaker wires and it seems to make the left channel come back on. Kinda strange.

Per the manual, the PTA4 is rated 20Wx2 @ 4-Ohm, 1% THD @ 1kHz; 32Wx2 @ 4-Ohm, 10% THD @ 1kHz; or 120W "max," whatever that actually means. Should be plenty good enough for a pair of PRO-7AVs, though.

The "wiggle the wires" problem, especially at low volume levels, usually means the speaker wire isn't making good contact inside the jack. Sometimes this can happen when the speaker wire is getting old and has excessive corrosion. The other thing to watch out for is that those speakers may be old enough to have breakup in the woofer surrounds.

ludi wrote:The "wiggle the wires" problem, especially at low volume levels, usually means the speaker wire isn't making good contact inside the jack. Sometimes this can happen when the speaker wire is getting old and has excessive corrosion. The other thing to watch out for is that those speakers may be old enough to have breakup in the woofer surrounds.

Oh, huh. I just cut and dejacketed this speaker wire. I wonder if I shouldn't re-do it.

"In the woofer surrounds"? If I'm understanding you correctly (and I'm probably not), wouldn't that affect the audio quality more than the volume or functionality (i.e. working at all)?

auxy wrote:"In the woofer surrounds"? If I'm understanding you correctly (and I'm probably not), wouldn't that affect the audio quality more than the volume or functionality (i.e. working at all)?

It would affect both. Seems that your speakers started life as an acoustic suspension design (superior design to my way of thinking) and ended up as a bass-reflex design. Not sure where your pair falls on that line. If yours are suspension-loaded, failure of the woofer (right, 4" is not a woofer) surrounds would break the air seal in the box and really trash the sound.

If you really like the speakers, finding someone to put new surrounds on the woofers isn't that hard. Paper and foam surrounds die quickest, while rubber surrounds tend to last a lot longer. Been down that path at least twice that I can remember.

Captain Ned wrote:It would affect both. Seems that your speakers started life as an acoustic suspension design (superior design to my way of thinking) and ended up as a bass-reflex design. Not sure where your pair falls on that line. If yours are suspension-loaded, failure of the woofer (right, 4" is not a woofer) surrounds would break the air seal in the box and really trash the sound.

If you really like the speakers, finding someone to put new surrounds on the woofers isn't that hard. Paper and foam surrounds die quickest, while rubber surrounds tend to last a lot longer. Been down that path at least twice that I can remember.

I see! Well, I don't actually, this is all way over my head, ahaha. The speakers still sound "great", though, from the perspective of an unwashed peasant like myself.

It seems "bass reflex" refers to speakers with a ported design. Mine have no such ports. They're sealed, and also apparently magnetically shielded. They're really heavy! (on the order of like 2kg apiece, which is heavy to a tiny person like me.) My dad said he bought them in the late 70s; they were a gift from him. They sound brilliant with the PTA4 at decent volumes, and get very loud long before I hit 2/3 of the dial's range.

OK, so they're the original suspension-loaded version. When the surrounds (the flexible bits linking the edge of the woofer cone to the frame) wear out and crumble into dust (they all do, eventually), sound quality goes downhill quickly. If the surrounds are intact, offer a bit of praise to the deity of your choice and keep the speakers out of the sun, as UV will kill anything if it's allowed to.

Here are some pictures to keep you awake at night. Sometimes the surrounds can break up by a third or more before the bass becomes really bad (and often buzzy). I've refoamed a half-dozen or so units. As the Cap'n says, rubber tends to last longest. Foam lasts maybe 10-15 years (depending on use and UV exposure) before starting to dry out and crumble.

I doubt your Optimus PRO-7AVs are as old as the 1970s. RadioShack sold several variations of a speaker in that format, but IIRC still had "Realistic" branding on all of its audio gear well into the 1980s. The "Optimus" branding picked up sometime in the late 1980s or early 1990s. The Kenwood amplifier, on the other hand, is definitely a vintage piece. Probably needs a good internal cleaning and some capacitor changeouts by now.

The 120W max rating reminds me of switching digital amplifiers (Class D) when operating in bridged mode (left and right channels combined). Bridged output is usually 2x the rating of stereo output combined. However, it could be some kind of "dynamic" wattage ratting, I suppose. If those specs are correct, I don't know how they get away with the 2x120W sticker on the front though.

I expect, for the money and size of the unit, they are using some sort of budget Class D amp section, something like this. This example is less than 4" square. All you need is one of these, a transformer and hopefully some sort of voltage regulation circuit with some beefy capacitors between the two.

ludi wrote:I doubt your Optimus PRO-7AVs are as old as the 1970s. RadioShack sold several variations of a speaker in that format, but IIRC still had "Realistic" branding on all of its audio gear well into the 1980s. The "Optimus" branding picked up sometime in the late 1980s or early 1990s.

Oooh. That's just what he told me. ¦3c

ludi wrote:The Kenwood amplifier, on the other hand, is definitely a vintage piece. Probably needs a good internal cleaning and some capacitor changeouts by now.

Maybe! I still have it. I got the Pyle to replace it, and it does an okay job. I decided I'm probably going to keep it.

The Kenwood is in very good shape physically, but there's lots of corrosion-dots inside from sitting out in our non-climate-controlled garage for a long time. It actually does still work decently, but the volume control knob is really scratchy, and if you don't put it in just the right spot, the left channel cuts out. Also, if you turn the volume up much, the audio starts to break up really bad. I actually spent a lot of time messing with tuner cleaner and DeoxIT trying to fix it, to no avail. :( I would *like* to fix it, but I'm not too handy with a soldering iron.

auxy wrote:It actually does still work decently, but the volume control knob is really scratchy, and if you don't put it in just the right spot, the left channel cuts out. Also, if you turn the volume up much, the audio starts to break up really bad.

That's an easy fix. The volume control is a potentiometer, a/k/a a variable resistor. It relies on direct copper-to-copper contact. Any old graunchy volume knob simply has dust/dirt getting in the way of the direct electrical connection. With the receiver POWERED OFF, violently swing the volume knob through its range. In most cases this is sufficient to kick out the chow getting in the way of the electrons. If this doesn't work, it's time to desolder the knob from the mainboard, take it out of the case, and have a party with some canned air.

Captain Ned wrote:violently swing the volume knob through its range. In most cases this is sufficient to kick out the chow getting in the way of the electrons.

Eheh, well, I did that, actually; it didn't help. And then I got all in there with tuner cleaner (helped a little) and a few weeks later, DeoxIT (did nothing). ¦3c

Captain Ned wrote:If this doesn't work, it's time to desolder the knob from the mainboard, take it out of the case, and have a party with some canned air.

Hehe. The nerds in Freenode ##electronics were suggesting I replace the potentiometer altogether, but I haven't bothered to do so yet. Like I said, it gets real crackly and the audio breaks up a lot when you turn it up even modestly, so I think there's something else more severely wrong; even if I replaced the volume potentiometer, I think it might neeed more work.

It was really dirty and dusty inside. I cleaned it out a little with my air compressor's spray nozzle, but it has a lot of some kind of dusty-dirty stuff clinging to all the parts. I thought about cleaning it out really well but I don't know if it would even help.

The crackly and break-up isn't a sign of larger things. It's just 30+ years of crap keeping the slider on the volume knob from directly contacting the underlying coil.

Replacing this will be dicey unless you get lucky and have a part number or electrical specs to order a new one with the exact same resistance profile.

The clingy dust is electrostatic in nature. All electronics accumulate this stuff and it's only an issue with parts that have moving pieces, like volume & balance controls and source selector controls.

Captain Ned wrote:The crackly and break-up isn't a sign of larger things. It's just 30+ years of crap keeping the slider on the volume knob from directly contacting the underlying coil.

Are you sure? I mean, you know way more about this than me, I have zero doubt -- I'm not questioning your knowledge or judgement at all! -- I just think I haven't quite been clear explaining what I mean.

The volume control is real crackly and noisy when you turn it, but if you get it to the "right" spot, the sound is very clear. However, if you turn it up a bit, even once you get it to the "right" spot, the output is broken-up and staticky, especially on bass-heavy music or any low sound (rumbling in video games). It only does this on the speakers; headphones are clear as day.

Captain Ned wrote:The crackly and break-up isn't a sign of larger things. It's just 30+ years of crap keeping the slider on the volume knob from directly contacting the underlying coil.

Are you sure? I mean, you know way more about this than me, I have zero doubt -- I'm not questioning your knowledge or judgement at all! -- I just think I haven't quite been clear explaining what I mean.

The volume control is real crackly and noisy when you turn it, but if you get it to the "right" spot, the sound is very clear. However, if you turn it up a bit, even once you get it to the "right" spot, the output is broken-up and staticky, especially on bass-heavy music or any low sound (rumbling in video games). It only does this on the speakers; headphones are clear as day.

That's why I was thinking it might be some other problem.

A crackly volume knob will only crackle as it's turned. What you're experiencing when you crank it up is clipping as you're pushing the amplification circuit beyond its capacity. The note that it happens with speakers but not with headphones is the clincher, as headphones require far less output power than speakers.

If the speakers are of 4-ohm impedance (it should say on the back by the terminals) there's a good chance that that old receiver simply wasn't designed to deal with 4-ohm speakers. The industry standard is 8-ohm impedance. 4-ohm speakers require twice the current (amperes) to create the same effective wattage. Old '70s receivers most likely don't have the oomph (highly technical term there) to push enough current to 4-ohm speakers.

Back in the early '80s there was a speaker (the Apogee Scintilla) whose impedance got as low as 1 ohm in the audible range (impedance varies with frequency and any good review or mfg's brochure will show the impedance/frequency curve). Driving this speaker was nigh-on impossible until Mark Levinson (brand name) came out with an amp (the ML-3) that could reliably double it's current output for each halving of impedance down to the 1=ohm level. An old review magazine, Audio, actually used one as an arc welder and didn't break it.

Captain Ned wrote:If the speakers are of 4-ohm impedance (it should say on the back by the terminals) there's a good chance that that old receiver simply wasn't designed to deal with 4-ohm speakers. The industry standard is 8-ohm impedance. 4-ohm speakers require twice the current (amperes) to create the same effective wattage. Old '70s receivers most likely don't have the oomph (highly technical term there) to push enough current to 4-ohm speakers.

Nooo, they're 8-ohm. It says "8Ω 40W MAX POWER 80W" on the back.

Heeyyy, since they're 8Ω, could that be why they're so loud with my PTA4? It's rated for 2x120W with 4Ω equipment...

Captain Ned wrote:If the speakers are of 4-ohm impedance (it should say on the back by the terminals) there's a good chance that that old receiver simply wasn't designed to deal with 4-ohm speakers. The industry standard is 8-ohm impedance. 4-ohm speakers require twice the current (amperes) to create the same effective wattage. Old '70s receivers most likely don't have the oomph (highly technical term there) to push enough current to 4-ohm speakers.

Nooo, they're 8-ohm. It says "8Ω 40W MAX POWER 80W" on the back.

Heeyyy, since they're 8Ω, could that be why they're so loud with my PTA4? It's rated for 2x120W with 4Ω equipment...

Nope. As the impedance increases current delivery decreases. Since watts = volts*amps, less current means less watts.

Captain Ned wrote:Nope. As the impedance increases current delivery decreases. Since watts = volts*amps, less current means less watts.

Oh. Argh, impedance is really confusing.

Impedence=resistance. The higher the impedance (in ohms), the more resistant the speaker's internal electronics are to the incoming signal. Back in the early days of hi-fi (i.e. '50s and '60s) most speakers had 16-ohm impedance as it was a whole lot easier and cheaper to build an amp that could push out voltage rather than one that can push out current (still a selling point for tube amps). It's why truly high-quality amps are all physically massive. Delivering current to low-impedance loads (4 ohms & under) requires so much current delivery capacity that the power supply transformers and capacitors in the amp must be epically large on their own.

Captain Ned wrote:Impedence=resistance. The higher the impedance (in ohms), the more resistant the speaker's internal electronics are to the incoming signal. Back in the early days of hi-fi (i.e. '50s and '60s) most speakers had 16-ohm impedance as it was a whole lot easier and cheaper to build an amp that can push out voltage rather than one that can push out current. It's why truly high-quality amps are all physically massive. Delivering current to low-impedance loads (4 ohms & under) requires so much current delivery capacity that the power supply transformers and capacitors in the amp must be epically large on their own.

So impedance = resistance. So you have voltage, current, and resistance. V=IR. Or, I = V/R, so assuming the voltage is constant, then as the resistance (or impedance) increases, current drops, and since the power P is equal to volts V times current I (P=VI), then as current drops, so does delivered power. Which you said. Basically.

So that means you need MORE current with a high-impedance device, right? Then why does it take so much current to run low-impedance loads? I've read this before and it's always confused me.

auxy wrote:So that means you need MORE current with a high-impedance device, right? Then why does it take so much current to run low-impedance loads? I've read this before and it's always confused me.

Because speakers/crossovers are voltage devices. Speaker output sensitivity is measured in how many dB are created by a 2.83V signal at 1 kHz. What changes is how much current the amp must push to reach the stated voltage level. A 4-ohm speaker will require 4 times as much current compared to a 16-ohm speaker to induce the same voltage level through the crossover and voice coils.

It's much easier for an amplifier to increase its voltage output than its current output. Increased voltage doesn't require massive transformers and capacitors, unlike increased current.

Captain Ned wrote:It would affect both. Seems that your speakers started life as an acoustic suspension design (superior design to my way of thinking) and ended up as a bass-reflex design. Not sure where your pair falls on that line. If yours are suspension-loaded, failure of the woofer (right, 4" is not a woofer) surrounds would break the air seal in the box and really trash the sound.

Yeah, but it wouldn't make the speaker cut out completely. His problem is electrical, not mechanical.

Captain Ned wrote:It's much easier for an amplifier to increase its voltage output than its current output.

To a point. On any amp that doesn't use transformer coupling of the output stage, your voltage swing is going to be limited to the voltage of the power supply rails.

The years just pass like trains. I wave, but they don't slow down.-- Steven Wilson

Captain Ned wrote:A 4-ohm speaker will require 4 times as much current compared to a 16-ohm speaker to induce the same voltage level through the crossover and voice coils.

THIS. This is the part that always confuses me. 4-ohm speaker, okay -- so if we're trying to reach 2.83v, then it's 2.83v divided by 4 ohms, or 707mA, and with a 16-ohm speaker it becomes 177mA... I just realized I've had the math backwards in my head the whole time. :lol: :math101: I'm an Asian who's bad at math, sue me. x3

Captain Ned wrote:It's also the rare solid-state amp that has output transformers, unlike tube amps where the rarity is a non-transformer output (seen a couple and they're bizarre beasties).

Yup. Transistors -- being low-voltage, low-impedance devices -- are electrically a better match for a speaker voice coil, so no transformer needed. I'm having a tough time imagining how you'd even make a transformerless tube output stage -- what do they do, use high-current radio transmitter tubes or something?

The years just pass like trains. I wave, but they don't slow down.-- Steven Wilson

just brew it! wrote:I'm having a tough time imagining how you'd even make a transformerless tube output stage -- what do they do, use high-current radio transmitter tubes or something?

You either build your own speaker with a really high impedance or you use a massive array of tubes. Either way OTL tube amps are notoriously fragile and they're all better space heaters than music amps.

EDIT: Here's a 2006 Stereophile review of an OTL amp complete with enough background to get a grasp on OTLs.